The present invention relates to a method of producing a composite steel body shaft used to form a shaft member such as a screw rotor having tooth portions and a shaft portion which screw rotor is used in a screw compressor, and relates more particularly to a process of producing a composite steel body shaft by electroslag remelting.
Japanese Patent Unexamined Publication No. 197232/1983 discloses an example of a method comprising the steps of providing a steel body with a cavity, inserting a consumable electrode into the cavity, melting the consumable electrode in the manner of electroslag remelting and thereafter solidifying the melt made of the consumable electrode, thereby manufacturing a composite steel body. Another example of the method, in which the consumable electrode is melted in the manner of the electroslag remelting, is disclosed in Japanese Patent Examined Publication No. 5402/1977. These methods were provided with a view to obtaining a high quality material having a fine structure. With respect to these related prior arts, no method of forming a composite steel body shaft by connecting a steel ingot 2 to the outer periphery of a central shaft member 1 made of the consumable electrode in the manner of electroslag remelting, as shown in FIG. 2, had been discussed.
Further, in the above-described prior art technique, only the connection of both a hollow steel body member and a consumable electrode material had been taken into consideration, that is, no forming of a shaft other than such connection by use of the electroslag remelting had been taken into consideration. Therefore, if a cross-sectional area of the cavity of the hollow steel body is smaller than that of the cavity of the metal mold, the upward movement of slag provided on a bath is obstructed by a steel body portion protruding radially inward from the periphery of the cavity of a larger diameter when a melting portion made of the material of the electrode reaches in the vicinity of the protruding steel body portion and when the protruding portion is to be melted, so that the slag mass cannot be smoothly raised and there is caused such a fear that a part of the slag is mixed with or confined in the remelting portion of the hollow steel body. This confined slag makes it impossible to obtain advantageous effects of the electroslag remelting technique which is used for forming a shaft portion formed in the hollow steel body so as to achieve high quality. Also, in an extreme case, there will occur a fear of serious defects in the end portion of the interface along which a shaft portion is integrated to the outer steel body, due to the confined slag.